This invention relates generally to carbon nanotubes, and more particularly to methods for aligning carbon nanotubes in the production of composite materials.
Carbon nanotubes have attracted the attention of materials scientists and engineers in connection with the production of composite materials, since their recent discovery. See, e.g., Dresselhaus, et al., in Graphite Fiber and Filament (M. Cardon, Ed.) (Springer, Berlin 1988). A carbon nanotube possesses unique physical properties with a very high length to diameter ratio, such that it is one of the strongest fibers known. Both single wall and multiwall nanotubes have very high Young's modulus, stiffness, and flexibility as demonstrated by experimental studies and theoretical modeling. See Tracy, et al., Nature, 381:678 (1996); Loirie, et al., J. Mat. Res. 13: 2418 (1998). The nanotube has an elastic modulus of about 1-2 Tpa, whereas that of an ordinary carbon fiber is about 750 Gpa. It is believed that this unique property can make carbon nanotubes highly useful in the production of composite materials and other ultrastrong materials. See Garg, et al., Chem. Phys. Lett. 295:273 (1998); Ajayan, et al., Adv. Materials 12:750 (2000); Ni, et al., Phys. Rev. B 61:R1613 (2000).
Uniform polymeric nanotube composites have been produced by a solution evaporation technique assisted by high-energy sonication, with thin homogeneous films (200 to 500 nm) produced by spin coating a few drops of the solution onto glass. See Stephan, et al., Synthetic Metals 108:139-49 (2000); Qian, et al., Appl. Phys. Lett. 76(20):2868-70 (2000). Nanotube composites can also be produced in the form of fibers. The fibers can be produced by extrusion (Andrews, et al., Appl. Phys. Lett. 75(9):1329-31 (1999)) or drawing (Kumar, et al., Polymer 43:1701-03 (2002)), where mechanical shear is used to orient bundles of nanotubes. It is difficult, however, to produce well-dispersed carbon nanotubes in a polymer, because the amount of nanotubes used in either the dispersion or the extrusion technique is very small compared to the amount of polymer. Aligned carbon nanotubes also have been produced in a polymer matrix using electrochemical synthesis. See Chen, et al., Appl. Phys. A, 73:129-31 (2001).
The ability to enforce a preferred alignment of carbon nanotubes in composites is important to draw anistropic behavior from them. See Bermejo, et al., J. Amer. Ceramic Soc. 78:365-68 (1995); Holloway, et al., J. Mat. Res. 8 (April 1993). Various techniques for aligning nanotubes have been reported, such as carbon arc discharge (Wang, et al., Appl. Phys. Lett. 62:1881 (1993)), clipping of epoxy resins (Ajayan, et al., Science 265:1212 (1994)), rubbing of films (de Heer, et al., Science 268:845 (1995)), chemical vapor deposition (Li, et al., Science 274:1701 (1996); Kyotani, et al., Chem. Mater. 8:2109 (1996); Fan, et al., Science 283:512 (1999)), and mechanical stretching of nanotubes in a polymer matrix (Jin, et al., Appl. Phys. Lett. 73:1997 (1998)). Magnetic orientation based on difference in magnetic susceptibility between the carbon nanotubes and the polymer has also been reported (Smith, et al., Appl. Phys. Lett. 77:663-65 (2000); Fujiwara, et al., J. Phys. Chem. 105(18):4383-86 (2001)). However, the magnetic susceptibility of both the polymer and the carbon nanotubes is very weak, on the order of 10-6 (Chauvet, et al., Phys. Rev. B 52(10):6963-36 (1995)), and requires high magnetic fields, e.g., about 15-25 tesla, to induce the orientation. Such a process is expensive and uneconomical for industrial applications, such as the manufacture of polymeric composites. It therefore would be desirable to provide improved and less costly methods for aligning carbon nanotubes. It would further be desirable to be able to use less powerful (i.e., producing weaker magnetic fields), more readily available and easy to use magnets to induce alignment of carbon nanotubes in making composite materials.
Several methods have been reported for chemical modification of nanotubes. These include silanization (Velasco-Santos, et al., Nanotechnology 13:495 (2002)) and electrochemical modification (Knez, et al., J. Electroana. Chem. 522:70 (2002)). It would be desirable to provide improved nanotube alignment techniques that avoid chemically modifying the carbon nanotube.